Silvering

A few pokes with a sharp pin and a generous coat of Micro Sol eliminated the remaining silvering, along with a matt varnish 'overcoat'. When it dried I restored the desired semi-gloss finish with a coat of satin varnish.

Once you have an overcoat on, additional overcoats are not likely to solve a silvering problem.  At this point you may have to sand/polish decals off and put on new ones.  That future overcoat even prevents using solvents or setting solutions.

I've had excellent results brushing on Future and then placing the decal on it. I then brush on another coat of Future. When dry, there is no silvering at all.

Also

Agree with Bish,,,    You also might want to poke a few holes in the silvering areas (use a new #11 blade) and apply some more decal sol....   a few well positioned, tiny holes will let the sol get underneath and suck the decal down even more.

Applying the Future over the decals makes no difference. Its whats under the dacls that matters. The sol will have pulled the decal down tighter onto the surface. Unless your aircrfat has a gloss finish, i would add you flat finish.

Just gave the decals a coat of micro sol, and the silvering has almost disappeared. When I finish the stencilling I'll give the whole thing a coat of Future and that should do the trick.